1. Field of the Invention
This invention relates to a method of producing a double oxide film, and more particularly to a lithium double oxide, a double oxide/metal composite and a functional conductive material including such a film.
2. Description of Related Art
Lithium double oxides are considered as electrode material in a lithium battery and its derivatives such as electrochromic device. Because the lithium double oxides have layered structure, lithium ions can be intercalated and deintercalated into the interlayer space. If the components of the lithium battery such as cathode, electrolyte and anode can be made in a film form, the lithium battery can be miniaturized even to micrometer scale. The major difficulty in fabricating lithium microbatteries is in the preparation of the lithium double oxide film as cathode. Therefore, the preparation of such a thin film has been attempted in various places and fields.
As a usual process, physical vapor deposition (PVD) process has been used. Such a process generally includes (1) the preparation of lithium double oxide powder or its precursor, (2) the formation of a target through shape-forming and sintering and (3) the deposition of a thin film from the target. For the deposition, the target is evaporated or ionized by sputter, laser ablation or similar methods, and deposited onto a substrate to form a thin film.
The PVD process is a multistage process and requires a high energy consumption for high temperature heating or ionization at each stage in addition to the energy consumption to keep a vacuum. Furthermore, even if the prepared target has a desired composition, the structure and chemical composition of the prepared film are not identical to those of the target due to the selective vaporization of Li upon the course of evaporation or ionization. Moreover, the life time of the target is short. Particularly, when the composition of the target is modified upon the evaporation or ionization, such a target is almost a waste. Therefore, this process is not an efficient way in views of energy and material at all.
In chemical vapor deposition (CVD) process as another usual process, organic metal complexes, halides or similar chemicals having high reactivity are mixed in vapor phase, transported and reacted onto a heated substrate. Generally, the starting substances and precursors are expensive and often toxic. In addition, the development of large scale apparatus is difficult because a vapor supplying system, a substrate transferring system, a heating system and an evacuation system must be placed in a closed container. Further, the quality of the resulting product is inferior to that of the product prepared by PVD process.
In other words, the fabrication of lithium double oxide thin film is far from practical use at present. Only a thick film obtained by dispersing and firing lithium double oxide slurry on metal film has been put into practical use on small size battery system, but not on microbattery system.